The present invention relates generally to the field of semiconductor packaging. More specifically, the present invention relates to an improved structure for organic ball-grid array (BGA) chip carriers having metal heat sink plates.
One of the popular BGA chip carriers available in the semiconductor packaging industry is an organic substrate utilizing cavity die-attach configuration. This type of chip carrier is typically constructed with an organic substrate having a multi-layer structure attached to a metal heat sink plate using an adhesive.
The organic substrate may be constructed from materials such as Bis-malesimide triazine epoxy (BT), FR4, polyimide, and polytetrafluoroethlyne. The metal heat sink plate (a.k.a xe2x80x9cheatslugxe2x80x9d) dissipates heat away from the semiconductor die and is usually made of a metal or metal alloy with relatively high thermal conductivity, such as Cu, Al, and Cuxe2x80x94W.
The organic substrate has an opening in the center that forms a die-attach cavity when the substrate is attached to the metal heat sink plate. A semiconductor die is placed within the die-attach cavity and attached to the metal heat sink plate using a thermally conductive adhesive such as silver epoxy. The die-attach cavity opening may typically be a square or a rectangular opening that would be appropriate to accommodate the particular die being attached to the substrate.
The semiconductor die is typically placed within the die-attach cavity with its active side facing out so that wirebond wires can form electrical interconnections between the die and the organic substrate. The wirebond wires connect bonding pads on the semiconductor die to corresponding bonding pads on the substrate and are typically ultrasonically bonded to the bonding pads. A glob-top epoxy sealant is then used to encapsulate the semiconductor die and the wirebond wires.
One of the concerns associated with chip carriers described above is that the difference in coefficients of thermal expansion (CTE) of the metal heat sink plate and the organic substrate results in warping of the chip carrier post-assembly. During the high-temperature curing step for the adhesive used to attach the organic substrate to the metal heat sink plate, the organic substrate expands more than the metal heat sink plate. As the adhesive cures, the organic substrate and the metal heat sink plate are fixated in this state where the organic substrate has expanded more than the metal heat sink plate. Upon subsequent cooling of the assembly, the organic substrate contracts more than the metal heat sink plate, causing the chip carrier assembly to warp.
Some examples of the adhesives that may be used to attach the organic substrate to the metal heat sink plate are: epoxy based adhesives, acrylic, and pre-preg. The curing temperatures for these adhesives are typically in the rage of 150-300 deg. C.
The chip carrier assembly warping poses manufacturability problems because under the current JEDEC standards for electronic packages, a package may not warp more than 0.008 inches. Although, the warping problem could be minimized or eliminated by increasing the thickness of the metal heat sink plate, thereby increasing the chip carrier assemblies"" stiffness, it is not practicable because also under the current JEDEC standards, the maximum thickness allowed for metal heat sink plates is only 1.0 mm, which is not sufficient to prevent chip carrier assemblies from warping.
To address the warping problem with organic cavity die-attach BGA chip carriers, the present invention utilizes a second organic substrate structure (the xe2x80x9csupplemental substratexe2x80x9d) to counter balance the bending force resulting from the mismatch of CTEs between the main organic substrate (the xe2x80x9cprimary substratexe2x80x9d) and the metal heat sink plate. The supplemental substrate is attached to the metal heat sink plate on the side opposite from the primary substrate resulting in a symmetrically stacked up structure with the metal heat sink plate sandwiched between the two organic substrate structures.
The supplemental substrate is preferably constructed from materials having a CTE that is substantially similar to the CTE of the primary substrate. The supplemental substrate preferably has as many physical characteristics of the primary substrate as possible and may also include a Cu core layer found in the primary substrate to better match the CTE of the primary substrate. More preferably, the supplemental substrate may be constructed from the same organic material as the primary substrate.
The supplemental substrate may also have a hole in the center that mirrors the die-attach cavity in the primary substrate. The hole exposes a portion of the metal heat sink plate for improved heat dissipation. If necessary, supplemental heat dissipating structures such as metal fins may be attached to the exposed portion of the metal heat sink plate.
Because the supplemental substrate, constructed from the same material as the primary substrate, has a CTE that is substantially similar to that of the primary substrate, the symmetrical stack up structure provides symmetry in the thermal expansion of the structures on the two sides of the metal heat sink plate, thus, preventing the assembly from warping.